run fortran code with vb.net gui

Question

hello.
I want to run my own fortran codes with a vb.net gui. if i run really simple codes with no input, no output files and no subroutines, it works. i use visual basic, and it is really simple to create a gui that can handle simple programs. i also can start programs like word, excel, ... with the files i want to open.

but i cannot run my own codes.
if my fortran code requires input / output files it won´t work. also i cannot run codes with subroutines. my big code has several subroutines in different .f files. how can i get this thing to work? i have no idea what i am doing wrong.
if i click on the .exe files of my fortran code, everything works just fine. i thought maybe it is because of a wrong directory somewhere in the .exe file. but it does not depend on the directory.

there is no help on the net. i think i am the only person in this world, who wants to create a gui for a little bit more sophisticated programs than "hello world".

i really appreciate your help on this. i cannot solve my problems by myselve.

thanks a lot.

greetings to all you experts out there.